The present invention relates to the field of direct AC/AC converter systems in general and more particularly to matrix converters, i.e. wherein the conversion is performed through a plurality of bilateral switches controlled for conduction according to a matrix of existence functions. See for instance U.S. Pat. No. 4,648,022 of C. D. Schauder. This patent is hereby incorporated by reference.
As specifically explained in the afore-stated U.S. patent, a static three-phase variable speed AC motor drive is supplied with the three AC output lines of an AC/AC converter system including three groups of three bilateral switches, wherein each group pertains to one output line and the switches of a group pertain to the respective AC input lines. Such a converter may be found, as a result of a rectifier function, the equivalent of a voltage source, i.e. a fictitious DC-link created from the input line voltage, and an inverter function performed thereupon with the result applied by the groups to the respective output lines. However, as a result from the fragmented operation so performed when combining the existence functions of the bilateral switches in sequence, the input current drawn from the power supply may be loaded with harmonics rather than being a pure sinusoidal wave. The object of the present invention is to control the bilateral switches, while under output current control for the motor drive, so as to preserve an input current of highest quality on the supply side of the AC/AC converter.
Attempts have been made in the past in order to improve the quality of operation of a direct AC to AC power converter. See: U.S. Pat. No. 4,468,725 of Marco Venturini; an article by Ziogas, Khan and Rashid entitled "Some Improved Forced Commutated Cycloconverter Structures" in IEEE Transactions Vol IA-21 September/October 1985 pp 1242 to 1253; an article by Ziogas, Khan and Rashid entitled "Analysis and Design of Forced Commutated Cycloconverter Structures with Improved Transfer Characteristics" in IEEE Transactions Vol. IE-33 August 1986 pp. 271 to 280; an article by Jose Rodriguez entilted "A Four Quadrant Three-Pulse Transistor Rectifier" in IEEE IAS 1984 pp 820 to 825; and a paper by Gunther Kastner and Jose Rodriguez entitled "A Forced Commutated Cycloconverter with Control of the Source and Load" presented at the First European Conference on Power Electronics And Applications in Brussels, on 16-18 October 1985 (Proceedings vol. 1 pp. 1.141 to 1.146).
PWM modulation schemes in this context for output voltage control have been shown in the afore-stated 1985 article by Ziogas, Khan and Rashid, and also in the U.S. Pat. No. 4,648,022 of Schauder. In particular, the Schauder patent uses a PWM technique applied to the rectifier function in order to eliminate the 5th and 7th harmonics in the AC input current.